Study On Eco-physiological And Pathological Responses Of Several Typical Plant Species To Elevated Atmospheric CO₂

Atmospheric CO2 concentration has rapidly increased from about 280 ppm pre-industry to more than 380 ppm nowadays, and is predicted to reach 730-1020 ppm by the end of this century. As CO2 is one of the substrate for plant photosynthesis, how plants would response to elevated CO2 becomes a main basis for disclosing the cycle and balance of carbon, as well as the global primary production in the context of future climate change. Though great progress has been obtained on this subject, there are still some shortcomings:Firstly, the existing researches have rarely considered responses of plant diseases. Secondly, the study on aquatic plants has largely lagged behind those on terrestrial plants. Thirdly, compared to the effects only by single elevated CO2, less is known of plant responses to the combined effects of elevated CO2 with other environmental factors. In this study, zucchini-powdery mildew(Cucurbita pepo-Podosphaera xanthii) pathosystem, grapevine-powdery mildew (Vitis vinifera L.-Uncinula necator) pathosystem, and water hyacinth(Eichhornia crassipes) were selected as research objects. By using phytotrons to simulate different environmental conditions, their responses to elevated CO2 (800 ppm) with rising temperature (4℃upper) or different nutrient levels (respectively as oligo-, meso-, eu-, and hypertrophic) were monitored. The main findings are as following:1. Elevated CO2 (EC) significantly enhanced the net photosynthesis rate (Pn) of zucchini plants (P<0.05), and produced more aboveground biomass and zucchini fruits than those in control treatment with 450 ppm CO2 (P>0.05). Elevated both CO2 and temperature (ECT) also enhanced Pn but with less enhancement (P<0.05), and greatly accelerated the plant development with more open leaves and male flowers (P<0.05). However, ECT reduced both chlorophyll content index (CCI) and area of zucchini leaf (P>0.05), finally resulting significantly decreased dry biomass accumulation (aboveground) and zucchini fruits production than the control treatment (P<0.05).2. Compared to the control, no evident effects on P. xanthii growth and development were found in the C. pepo-P. xanthii pathosystem with EC, and the plant disease index (DI) slightly reduced because of the improved plant resistance due to increase of atmospheric CO2. However, the fungal development of P. xanthii with ECT greatly improved, with significantly greater colony size and conidiophore production (P<0.01), as well as much heavier DI (P<0.01) with much lower yield of zucchini fruits (P<0.01).3. At earlier days after inoculation, EC greatly improved CCI and Pn of grapevine (P<0.05), while ECT also enhanced CCI even with more enhancement and Pn but with less enhancement. But grapevine plants with both EC and ECT treatment showed photosynthetic acclimation, with Pn under ECT even lower than that under control. The stomatal conductance of grapevine was reduced by EC, and it was reduced more when with higher temperature or powdery mildew infection.4. The disease incidence and disease severity showed nonsignificant decrease toward CO2 enrichment, whereas they generally showed significant increase under ECT (P<0.05). Finally, our result also suggested that although the two varieties of grapevine showed similar response trend to environmental conditions, the extent was varied, with Barbera probably being more subjectable.5. Overall, EC consistently enhanced photosynthesis and growth of water hyacinth at all nutrient levels. The positive effects due to EC could even overbalance the inhabitation by nutrient decline between narrow ranges, whereas it could only partly compensate the inhabitation by nutrient decline between wide ranges. Moreover, the enhancement extent caused by elevated CO2 varied among nutrient availabilities, being more in eu-and hypertrophic levels and less in meso-and oligotrophic levels. Further, the plant with relative suit nutrient would show higher CO2-induced improvement of Pn and biomass than other ordinary plants.6. E. crassipe showed flexible plasticity to EC. The CO2-enrichment deduced assimilation was allocated more to plant roots than shoots which would improve the nutrient absorption capacity, and mostly transferred to offspring ramets rather than maintained at the mother ramet which would benefit the vegetative reproduction. All these flexible responses would be helpful for its invasion.7. Under EC, the nitrogen (N) and phosphorus (P) contents of E. crassipe slightly decreased. This might be resulted from a dilution effect by more CO2-induced assimilation, and/or improved nutrient use efficiency under EC. This finding suggests that there might be increased difficulties in preventing infestation of E. crassipe by reducing N and/or P in eutrophic waters. Nevertheless, the total N and P accumulation per plant increased suggesting higher bioremediation efficiency of using water hyacinth for water eutrophication.In summary, our results indicate that in future climate characterized by both EC and rising temperature, the powdery mildew would cause more damage on both zucchini and grapevine. There could be more water hyacinth infestations in natural waters benefited by elevated atmospheric CO2. More studies are needed to better understand and manage those crops in a future changing climate.